add IndexStatsHelper

Former-commit-id: 6740cb7b42af16d7269b071798eef02995031c26

cpp/CHANGELOG.md

@@ -37,6 +37,7 @@ Please mark all change in change log and use the ticket from JIRA.
 - MS-260 - Refine log
 - MS-249 - Check machine hardware during initialize
 - MS-261 - Update faiss version to 1.5.3 and add BUILD_FAISS_WITH_MKL as an option
+- MS-278 - add IndexStatsHelper
 
 ## New Feature
 - MS-180 - Add new mem manager